09--three-sum.java

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> triplets = new ArrayList<>();
        for (int i = 0; i <= nums.length - 3; i++) {
            if (i == 0 || nums[i] != nums[i - 1]) {
                int left = i + 1, right = nums.length - 1;
                int target = 0 - nums[i];
                while (left < right) {
                    if (nums[left] + nums[right] == target) {
                        List<Integer> triplet = new ArrayList<>();
                        triplet.add(nums[i]);
                        triplet.add(nums[left]);
                        triplet.add(nums[right]);
                        triplets.add(triplet);
                        
                        while (left < nums.length - 1 && nums[left] == nums[left + 1]) left++;
                        while (right > 0 && nums[right] == nums[right - 1]) right--;
                        left++;
                        right--;
                    } else if (nums[left] + nums[right] < target) {
                        left++;
                    } else {
                        right--;
                    }
                }
            }
        }
        return triplets;
    }
} // TC: O(n log n) + O(n^2), SC: O(1)